The objectives of the experiment were to determine the bioavailability of tryptophan in soybean meal for 4-wk-old ducks using the slope-ratio bioassay with L-tryptophan as the reference diet and estimate the efficiency of dietary tryptophan retention in the carcasses of 4- to 7-wk-old ducks. An initial representative group of 12 ducks with an average body weight of 1,951 g (SD = 69) were killed for carcass compositional analysis. A basal diet formulated to contain 1.13 g of tryptophan/kg and adequate in all other amino acids, was supplemented with 0.2, 0.4, or 0.6 g tryptophan/kg from L-tryptophan, or 0.2 or 0.4 g tryptophan/kg from soybean meal. Four pens of three 4-wk-old ducks received each of the six diets for 21 d. Ducks fed the basal diet and the basal diet supplemented with 0.2 or 0.4 g of L-tryptophan/kg for 21 d were used as the final slaughter groups for carcass compositional analysis. Dietary supplemental tryptophan from either L-tryptophan or soybean meal linearly increased final weight (P < 0.05), weight gain (P < 0.10), and gain:feed ratio (P < 0.05). Common-intercept, multiple linear regressions in slope-ratio methodology performed using weight gain and gain:feed ratio as dependent variables and grams of supplemental tryptophan/kg of diet as the independent variable gave 94 and 92% tryptophan bioavailability in soybean meal relative to L-tryptophan, respectively. Linear regression of tryptophan deposition in the carcass on tryptophan intake resulted in a 21% efficiency of carcass tryptophan retention above maintenance.